Embodiments of the invention relate generally to electric machines and, more particularly, to a sensorless method for controlling an electric machine via the use of signatures that are produced by the saturation behavior of the electric machine dependent on an alignment of excitation current provided to the stator thereof.
The usage of electric machines in various industries has continued to become more prevalent in numerous industrial, commercial, and transportation industries over time. With an electric machine, be it an interior electric machine (IPM) machine, permanent magnet (PM) assisted synchronous reluctance machine, or a synchronous reluctance machine, position determination of the rotor or the magnetic field is a critical informational element for torque control of the machine.
For purposes of determining rotor position in an electric machine, an encoder, tachometer, or resolver may be used as the position sensor. For measuring the magnetic field position in the electric machine, magnetic field sensors such as Hall effect sensors may be used. The sensors/devices utilized for determining the rotor position and magnetic field may be used in combination with one another, with it being recognized that position sensor or rotor position sensors and magnetic field sensors for measuring magnetic field orientation or strength may both be employed for the purpose of electric machine control. It is recognized, however, that the position sensor (e.g., encoder), along with its cabling and interface electronics, contributes to a significant portion of the motor drive system cost and overall complexity and is often a major reliability concern. The cost, complexity, and reliability issues associated with the use of position sensors in determining rotor position has led to the advent of systems and methods of sensorless position sensing and control of electric machines (i.e., not requiring rotor position or speed sensors). Because of the tremendous potential of cost reduction and reliability improvement, sensorless control has been a major research challenge. Most of the sensorless control methods that are available in industries are so called Back-EMF tracking approaches. Back-EMF tracking methods utilize the voltage of the motor winding induced by the time variation of the flux-linkage caused by the rotation of the rotor. These methods perform very well near the rated speed where the back-EMF voltage is close to rated voltage. However, as the speed is reduced, the back-EMF magnitude is reduced and performance is deteriorated. Eventually, as the speed is close to zero, these methods become unstable and fail, because the back-EMF diminishes and becomes unobservable. This limits back-EMF tracking methods to HVAC type of applications where open-loop starting is acceptable.
Owing to its tremendous potential of cost reduction, reliability improvement, and elimination of interfaces, zero and near zero speed (or frequency) sensorless control has been a major research challenge for decades, and high frequency injection methods have been the most promising solution for the sensorless operation on or near zero frequency for AC electric machines with saliency. High frequency signal injection is used to track the rotor angular position and velocity of various AC electric machines having a rotor providing an impedance that varies with rotor position or flux position, such as described in U.S. Pat. Nos. 5,585,709; 5,565,752; 6,131,258; 6,069,467; 5,886,498; and 6,639,380, for example. In employing a high frequency signal injection technique, small signal saliency and small signal saliency angle is the key information used for sensorless control, with such saliency being defined using small signal impedance.
However, with respect to previously employed encoderless controls that employ high frequency injection, it is recognized such encoderless controls have failed to find success in recovering the full, or near full, torque capability of the machine due to the loss of small signal saliency at high-load levels for the machine, this being due to magnetic saturation at such high-load levels. That is, as the torque level (thus the current level) is increased, the q-axis starts to saturate and eventually small signal inductance of the q-axis becomes lower than the d-axis inductance, thereby making saliency tracking sensorless control infeasible. Moreover, a cross saturation effect causes the saliency angle to shift away from the d-axis, causing position estimation angle error even though the q-axis small signal inductance is still larger than d-axis. Therefore, existing high frequency injection methods have been limited to applications where torque density requirement is low and also the dynamic performance requirement is modest. Especially, for such applications as traction drives where very high torque density is desired, existing sensorless control methods could achieve only a small fraction of the desired torque requirement, let alone the dynamic performance.
In an effort to address the saliency tracking limitations of the previously described high frequency injection method for zero frequency encoderless control, efforts have been made to incorporate design features into an electric machine (e.g., IPM machines or synchronous reluctance machine) that enable increased torque control without the use of any position sensor. That is, a special rotor structure—i.e., a “D-ring”—that increases magnetic saliency for high frequency excitation has been incorporated into the electric machine, wherein this high frequency excitation can be used for sensorless/encoderless motor control. The rotor structure introduces electrical circuits (shorted circuit, closed circuit with passive or active elements) to a specific orientation of the rotor so that it couples with the stator winding magnetically. The position of the rotor is measured by applying a high frequency carrier voltage to the stator and by indirectly measuring the current of the rotor, by measuring the (reflected) high frequency carrier current response in the stator. If the rotor circuit is aligned in phase with the high frequency injection, the impedance of the motor is reduced. This variation of impedance is used to track rotor position. As a result, small signal saliency up to necessary loading level is introduced and maintained without impact on the electric machine performance, efficiency, and reliability.
While the inclusion of such a special rotor structure or D-ring has enabled increased torque control of an electric machine via the use of high frequency injection sensorless controls, it is recognized that inclusion of such a D-ring in the electric motor may still not provide for full sensorless control of an electric machine, as even with a D-ring it is difficult to maintain a desired level of saliency during operation of the machine due to the severe saturation at very high torque level.
Therefore, it is desirable to provide a system and method for sensorless control of an electric machine that allows for control over a full operating range of speed and torque. It is further desirable for such a system and method to provide such sensorless control in a manner that overcomes the shortcomings associated with saliency tracking based control methods, such as the effects of magnetic saturation and phase error in position tracking and control of the machine.